Write once read many times (WORM) memory devices may be employed for storage of data or multimedia such as music or video. Moreover, for large-volume, disposable RFID applications, write-once-read-many-times WORM memory is sufficient. Radio frequency identification (RFID) tags are an attractive application for printed organic circuits (1), especially when the circuit can be printed directly on the same substrate as the antenna used to power the circuit (2-4). RFID tags require a memory function which can be programmed and read using the power taken from the radio frequency field, needing very low current consumption and operating voltage.
Moller et al (5) discloses a solution-processed WORM device based on polyethylenedioxythiophene doped with polystyrene sulfonic acid (PEDOT:PSS), wherein a PEDOT:PSS thin film is deposited on a p-i-n silicon structure. Voltage pulses (8-10 V) and large current densities (of the order of hundreds of amps per square centimeter) are needed to program the memory arrays of this device by irreversibly reducing conductivity of the PEDOT:PSS layer. Moller et al discloses that the current density remains reversible at 0-2 V and that permanent conductivity changes are small (ca. 3-5 times lower) at 4V.
US 2005/157535 discloses a WORM device comprising an organic polymer layer and an electron blocking layer to inhibit electrons from entering the organic-polymer layer and in order to prevent degradation of the organic polymer layer due to repeated read access.
Brito et al. discloses a low-power WORM memory device in patterned micro-holes (4), but the lithography process raises the cost and restricts the applicability of this type of device.
It is therefore an object of the invention to provide a WORM memory device having a memory array that may be programmed at a lower current density and/or voltage.
It is a further object of the invention to provide a WORM device that may be manufactured at low cost.